A variety of types of electric motors are known that make use of a Halbach array magnet. For example, the use of a magnet in an application involving a linear motor is disclosed in JP-A-2003-209963, and the use of a magnet in an application involving a rotor is disclosed in JP-A-2004-350427. In the linear motor of JP-A-2003-209963, a Halbach array structure (Halbach magnet array) is employed as a field pole magnet array. In the rotor of JP-A-2004-350427, the use of the Halbach array structure results in a reduction in thermal demagnetization, an increase in the effective magnetic flux, and other improvements.
The Halbach array structure is composed of a main magnet whose polar orientation (N pole, S pole; referred to below as the “magnetization direction”) is a first direction, and a submagnet whose magnetization direction is a second direction substantially perpendicular to the first direction. The main magnet is composed of a first main magnet and a second main magnet. The submagnet is composed of a first submagnet and a second submagnet. The magnets are permanent magnets. In the Halbach array structure, the first main magnet, first submagnet, second main magnet, and second submagnet are repeated as a basic arrangement in the stated order until a required length or shape has been achieved. An overall configuration of a magnet assembly of a motor rotor is thereby formed. In the above magnet assembly arrangement state, the magnetization directions of the first main magnet and second main magnet, which are arranged in an alternating fashion, are oriented in the first direction and face opposite directions. Similarly, the magnetization directions of the first submagnet and second submagnet, which are arranged in an alternating fashion, are oriented in the second direction and face opposite directions.
In a rotor for an electric motor having a magnet assembly based on the above-described Halbach array structure, magnetic flux density is increased by the array structure of the magnets, the magnetic flux is efficiently utilized, and a small-sized high-performance rotor can be realized without the need for a yoke for forming a magnetic path.
Over the past several years, the performance of magnets has improved, and magnets having extremely strong magnetic forces have become prevalent. However, if a Halbach array is used in which a plurality of magnets having different magnetization directions is densely arranged, a strong cover or large guide must be placed around the magnets in order to prevent the magnets from moving out of alignment due to repelling forces between adjacent magnets. In addition, a structure is utilized in MRI and the like in which the magnets are slightly separated and held by a guide because importance is placed on securely fixing the magnets. On the other hand, wind-power generation motors, elevator-driving motors, in-wheel motors (directly installed in a wheel of a vehicle), and other electric motors must be assembled to fit into a narrow space. However, a large guide or cover presents an obstacle to fulfilling this requirement. When the magnets are arranged so as to be separated from one another, the magnetic flux density will decrease, and the performance of the motor will therefore decrease. More specifically, when the magnets are arranged in a separated fashion, magnetic resistance in the magnetic circuit formed by the magnets will increase, the effective magnetic flux that is interlinked with a coil and that contributes to the motor output will decrease, and the performance of the motor will therefore decrease.